Low temperature heat seal films and foils are used to package food products and consumer goods. In one common application, low temperature heat seal films and foils are run on high speed, form, fill and seal packaging machines. The film or foil is typically coated with the heat seal resin on one surface of the web. As the web is fed through the machine, the film or foil is folded to expose the resin covered surfaces face to face, and then the films are sealed along seams using pressure and heat, for example by pressing the films together between heated platens. In addition, the film can also be heat sealed to different packaging substrates such as food container lidding. The performance of the seal in terms of strength and package integrity is largely a function of the characteristics of the film or foil substrate and the heat seal layer, as well as machine operating conditions such as platen temperature, pressure, and dwell time. Since these films are used in food or pharmaceutical packaging, it is desirable that the heat seal coating, as well as the packaging film or foil, be FDA direct food contact compliance (21CFR 175.320 and/or 21CFR 175.300).
Current technology for low temperature heat seal films includes films with water-based or solvent-based coatings, extrusion coated films and co-extruded polyolefin films. Co-extruded or extrusion coated products typically use films such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra low linear density polyethylene (ULLDPE), ethylene vinyl acetate (EVA) or ionomer technology. Co-extruded and extrusion coated products require expensive film extrusion equipment to manufacture. Companies making these heat seal films normally sell the films in roll form. The rolls are printed, and then supplied in roll form for use on form, fill and seal packaging machines.
Currently, solvent and water-based coatings typically use polyvinylidene chloride (PVDC), acrylic, or ethylene acrylic acid (EAA) technology. It is important that the heat seal coating anchor securely to the base film or foil in order that the resulting heat seal film or foil exhibits adequate bonding performance and acceptable shelf life. Many of these solvent and water-based coating formulations require priming in order to properly prepare the base film for suitable anchorage of the coating. The need for the priming the base film or foil prior to applying the heat seal coating renders these coatings economically impractical for use by many film converters. An objective of the present invention is to provide a formulated heat seal coating that is suitable for use by converters printing packaging films or foils for the food industry, and to provide such a formulation that does not require priming in order to achieve appropriate anchorage to films and foils commonly used in the food industry. At present, most converters are forced to buy expensive low temperature, heat seal films or foils sold by film suppliers.
Another purpose of the present invention is to provide an effective low-temperature heat seal formulation, which enables converters to affordably coat packaging films or foils with a low temperature heat seal coating. The term “low temperature, heat seal coating” is used herein to refer to a coating that is able to seal onto itself at a temperature as low as 70° C. with bond strength in excess of 300 gli (grams per linear inch) and also can be sealed to other packaging materials at 90-130° C., such as food container lidding and trays.
U.S. patent application Ser. No. 11/546,672 describes a self-priming, water-based, heat seal coating for packaging films. The formulation in the '672 patent application is based on copolymers of ethylene and acrylic or methacrylic acid blended with an aliphatic polyurethane emulsion which serves as an adhesion enhancer. Published data regarding this product shows it to have a narrow processing window and lower bond strengths than the current invention.
U.S. Pat. No. 6,607,823 discloses another water-based copolyester heat seal coating using 1,3-propane diol, isophthalic acid, and a sulfomonomer. The coating solution has 1-30 weight percent solids. The coating can be applied to polyamide, polyolefin, and polyester films using conventional off-line coating processes with corona treatment. The heat seal temperature is between 110-170° C. The dwell time is about 0.5-10 seconds under 20-60 PSI pressure. It has good self seal, about 4 pounds per inch peel value.
U.S. Pat. No. 6,543,208 teaches a food packaging film made of three layers: an inner low melting point polyester sealant layer, a vapor deposited ceramic or metal layer, and an outer side high melting point polyester layer. A food bag is formed by heat sealing the film with the low melting point sealant layer inside. The melting point of the sealant layer is below 160° C. One sealant layer from Eastman Chemical has a melt point as low as 80° C. and another sealant layer has a melting point of 122° C. The heat sealable layer is laminated to the metalized PET film using a laminating adhesive. The food bag is made with a high speed packaging machine. The seal bar temperature is about 180-200° C.
U.S. Pat. No. 8,079,470 discloses a co-extruded PET film with one side having an amorphous polyester heat seal layer and the other side having a polyethylene copolymer heat seal layer. The double side sealable film is heat sealed to the blister packaging containers for CDs and DVDs. The polyethylene copolymer layer has a heat sealing temperature of 65-150° C. under 5-80 psi sealing pressure. However, there is no detailed description of the amorphous polyester heat seal condition.
U.S. Pat. No. 8,389,117 describes a polyester based hot melt adhesive for roll applied labels. The hot melt polyester adhesive used 1,4-cyclohexane dicarboxylic acid, 1,4-cyclohexane dimethanol, triethylene glycol, and diethylene glycol. Tackfier, plasticizer, and nucleator were used to improve adhesion, adjust heat seal temperature, and speed up crystallization process. The molecular weight of the polyester is between 1,000 and 15,000. The melt viscosity of the polyester resin is between 300 and 3,000 centipoise at 150° C. The shrink label is applied about 80-90° C. and the residence time is between 2 and 20 seconds.
In contrast to the above patents, the present invention is directed to a solvent-based, heat seal coating and its use by film converters. The solvent-based, heat seal coating exhibits strong bond strength over a wide range of processing temperatures including relatively low sealing temperatures and different film substrates.